A winter tyre is supposed to ensure good traction when used on snow and ice. However, a winter tyre also needs to provide good driving characteristics when used on dry or wet roads.
Winter or snow tyres have been designed with treads having traction blocks widely spaced by large and deep grooves that allow a good traction and brake on snow. The presence of large and deep traction grooves increases the void-to-rubber ratio of the tread, also simply named void ratio i.e., the amount of open space compared to the amount of rubber in the tread. A low void ratio means more rubber in contact with the road and vice versa. The high void ratio tread pattern is excellent in deep snow because snow is trapped in the void spaces and increases the grip. However, these tread patterns reduce the stiffness of the tread and generate substantial noise and block mobility when used on wet or dry roads. Moreover, the tread wear rate is rapid and therefore these tyres are considered for use only during the snowy winter months. Additionally, these tyres are not particularly well suited for icy road conditions.
The use of treads having a high density of transverse sipes made on the blocks has provided a good improvement to both snow and ice traction performance. Such heavily bladed tyres can exhibit good winter ice traction performance by providing an increased number of tread edges to provide forward traction. However, an increased number of sipes again promotes a reduction of the stiffness of the tread with an increase of block mobility and noise during use.
Further, the performance of a tread on wet roads is increased by the presence of grooves able to drain water from the contact surface of the tread with the road. This is accomplished in the art by longitudinal or transverse grooves. The wider the groove, the higher the amount of drained water. However, broad longitudinal grooves reduce the traction on snow and broad transverse grooves reduce the stiffness of the tread, again increasing the noise and block mobility when driving on dry road.
In view of the above, it is clear that a winter tyre needs to satisfy several conflicting requirements in order to provide good performances on all the various road and weather conditions.
WO02/068221 and WO02/068222 in the name of the same Applicant disclose winter tyres comprising transverse grooves and two or three circumferential grooves defining a plurality of blocks disposed in central and shoulder rows. The transverse grooves have a curvilinear profile converging on the equatorial plane in the direction of rolling specified for the tyre. The frontal profiles of the axially adjacent portions of the circumferentially adjacent block comprise at least two successive curvilinear portions which are different from each other and are designed to form means of attenuating the noise arising from the contact of the tread pattern with dry roads when the tyre is in the rolling condition.
U.S. Pat. No. 5,435,366 discloses a pneumatic tyre with tread having good aquaplaning performance and good winter characteristics comprising tread blocks of a central block row separated from one another by inclined grooves which consist of two angularly extending groove sections which meet at the central circumferential plane. Furthermore, two circumferential grooves extend at an acute angle to the circumferential plane of the tyre in the region of the blocks which bound them at the sides.
EP 661,181 discloses a pneumatic tyre comprising a main groove extending in the circumferential direction of a tyre and lateral grooves extending in the axial direction of the tyre, the lateral grooves intersecting the main groove to divide the main groove into a plurality of main groove parts each extending between the circumferentially adjacent intersections of the lateral grooves with the main groove, the main groove parts having at least two different circumferential pitch lengths, a short one and a long one. The characteristic tread pattern disclosed in such a patent provides a pneumatic tyre in which the wandering phenomenon caused by the lateral rain grooves is prevented and the running noise due to air resonance in the main grooves is effectively reduced without sacrificing wet performance.
European Patent No. 139,606 and U.S. Pat. No. 5,088,536 disclose tread patterns which provide acceptable all season performance while maintaining ride, noise and handling characteristics.
The Applicant has found that the winter tyres disclosed in the above patent references suffer of an excessive flexibility of the blocks which worsen the driving performance on dry or wet roads, in particular when steering at medium and high speeds, and can be improved having regard their performance on snow and ice.
The Applicant has perceived that in spite of the efforts made to improve the overall characteristics of a winter tyre, there is still the need to provide a winter tyre which combines good grip on snowy or iced roads with good driving performance on dry or wet roads and an acceptable level of noise and wear.
The Applicant has found that such a combination of characteristics can be advantageously achieved by providing a tyre with a tread pattern comprising: 1) at least two circumferential grooves, which define two shoulder regions, namely a first and a second shoulder region, and at least one central region, 2) a plurality of asymmetric transverse grooves having a substantially “V” shape, which substantially extend for the whole width of the tread, said plurality of asymmetric transverse grooves comprising an alternate sequence of a first asymmetric transverse groove and a second asymmetric transverse groove defining an alternate sequence of a first asymmetric module and a second asymmetric module, both the first and second asymmetric modules extending in said two shoulder regions and said at least one central region, and 3) a plurality of lateral transverse grooves, which comprises at least one first lateral transverse groove substantially extending for the whole width of said first shoulder region and for a portion of said at least one central region of said first asymmetric modules, and at least one second lateral transverse groove substantially extending for the whole width of said second shoulder region and for a portion of said at least one central region of said second asymmetric modules.
Each sequence of a first asymmetric module and a second asymmetric module forms a main module resulting from the combination of the first asymmetric module and the second asymmetric module. Accordingly, the alternate sequence of first asymmetric modules and second asymmetric modules defines a repetitive sequence of such main modules.
According to the present invention, the plurality of asymmetric transverse grooves having a substantially “V” shape, together with the plurality of lateral transverse grooves, both crossing said at least two circumferential grooves, define in each sequence of said first asymmetric module and said second asymmetric a tread pattern wherein the shoulder portions comprise a sequence of n blocks and the central region comprises a sequence of n−1 blocks.
The Applicant has found that the above described tread pattern according to the invention allows to reduce the flexibility of the tread elements and the rolling noise, so improving the performance on dry roads, to provide a good drainage of water and a good grip on wet roads, and to have at the same time an effective grip on snow and ice.
In a preferred embodiment, said asymmetric transverse grooves having a substantially “V” shape are formed by a first and a second linear groove having a different length each other, each linear groove being preferably formed by an external portion and an internal portion having different orientation. The external portion forms an angle β having an absolute value of from 0° to 20°, more preferably from 0° to 15°, and most preferably from 0° to 10°, with respect to the radial plane of the tyre. The internal portion forms an angle α having an absolute value of from 15° to 65°, more preferably from 30° to 60°, and most preferably from 35° to 55°, with respect to the radial plane of the tyre. Accordingly, the internal portions form each other a central overall angle ranging from 50° to 150°. The asymmetric transverse grooves having a substantially “V” shape have the vertex in said at least one central portion of the tread and axially offset with respect to the equatorial plane of the tyre. First asymmetric transverse grooves have the vertex between the first circumferential groove and the equatorial plane of the tyre. Second asymmetric transverse grooves have the vertex between the second circumferential groove and the equatorial plane of the tyre.
Said lateral transverse grooves are preferably formed by an external portion and an internal portion having different orientation. The external portion forms an angle β having an absolute value of from 0° to 20°, more preferably from 0° to 15°, and most preferably from 0° to 10°, with respect to the radial plane of the tyre. The internal portion forms an angle α having an absolute value of from 25° to 65°, more preferably from 30° to 60°, and most preferably from 35° to 55°, with respect to the radial plane of the tyre. Said lateral transverse grooves end in said at least one central portion. The first lateral transverse grooves preferably end between the first circumferential groove and the equatorial plane of the tyre, but can also extend until to cross the equatorial plane and to end between the second circumferential groove and the equatorial plane of the tyre. The second lateral transverse grooves preferably end between the second circumferential groove and the equatorial plane of the tyre, but can also extend until to cross the equatorial plane and to end between the first circumferential groove and the equatorial plane of the tyre. The number of lateral transverse grooves provided in each first asymmetric module and second asymmetric module is not particularly limited and depends, on a matter of choice, from the number and dimension of the modules, from the number and distance of asymmetric transverse grooves and from the dimension of the tread. Preferably, the number of lateral transverse grooves presents in each first and second asymmetric modules is lower than five, more preferably lower than three, and still more preferably two or one.
The Applicant has found that the implementation of the above mentioned preferred embodiment can improve the water drainage when driving on soaked road and can improve the directionality and reduce the wear.
According to a preferred embodiment, the tread pattern further comprises a plurality of transverse sipes. The plurality of transverse sipes can be formed in said shoulder regions and/or said at least one central region of each of said first and second asymmetric modules. More preferably, the plurality of transverse sipes are formed in said at least one central region.
According to a further preferred embodiment, each of said first and second asymmetric modules comprises at least one transverse sipe formed in said at least one central region continuously joined to a corresponding sipe formed in at least one of said shoulder regions by a corresponding sipe formed in at least one of said circumferential grooves.
According to a more preferred embodiment of the present invention, each of said first and second asymmetric modules comprises at least one transverse sipe extending continuously from one shoulder region to the opposite shoulder region.
The Applicant has found that the implementation of the above mentioned preferred embodiment can further improve the effectiveness of the sipes in the traction and the grip on snow and/or ice. On the other hand, a lower number of sipes can be formed to obtain the same traction and grip of the tread on snow and/or ice. Consequently, the stiffness of the tread elements is increased so giving better performances in terms of rolling noise, driving stability and tread wear when driving on dry road.
For the purpose of better understanding the present invention, the term “asymmetric” refers to a pattern formed on the surface of the tread which is asymmetrical with respect to the equatorial plane of the tyre; the term “equatorial plane” means the plane perpendicular to the tyre axis of rotation and passing through the center of its tread; the term “radial plane” means a plane passing through and containing the tyre axis of rotation; the term “groove” means an elongated void area in a tread that may extend circumferentially or transversely; the term “sipe” means a thin cut formed into a tread.
Unless otherwise indicated in the present description, each angle—which is used to indicate the inclination of a given tread pattern element—is intended to be calculated as the angle defined between a radial plane of the tyre and the plane said given tread pattern element belongs to.